U.S. patent number 7,375,827 [Application Number 11/041,520] was granted by the patent office on 2008-05-20 for digitization of undercut surfaces using non-contact sensors.
This patent grant is currently assigned to Optimet, Optical Metrology Ltd.. Invention is credited to Gregory Agronik, Karol Sanilevici.
United States Patent |
7,375,827 |
Sanilevici , et al. |
May 20, 2008 |
Digitization of undercut surfaces using non-contact sensors
Abstract
An apparatus and method for digitization of surfaces of complex
objects such as dental surfaces or plastic parts. While an object
is being scanned by translation along a known trajectory, typically
in a plane, the line of sight of a distance probe is bent toward an
object by a periodic series of reflecting surfaces characterized by
normal vectors at distinct non-orthogonal angles with respect to
their axis of symmetry. One or more reference objects are scanned
using the same translation and mirror positioning systems. Gathered
coordinate data are processed to apply angular corrections and
combined to form a single distortion-corrected image.
Inventors: |
Sanilevici; Karol (Jerusalem,
IL), Agronik; Gregory (Jerusalem, IL) |
Assignee: |
Optimet, Optical Metrology Ltd.
(Jerusalem, IL)
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Family
ID: |
35456378 |
Appl.
No.: |
11/041,520 |
Filed: |
January 24, 2005 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050234344 A1 |
Oct 20, 2005 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60544468 |
Feb 13, 2004 |
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Current U.S.
Class: |
356/614;
356/622 |
Current CPC
Class: |
G01B
11/24 (20130101); G03H 2222/31 (20130101) |
Current International
Class: |
G01B
11/14 (20060101) |
Field of
Search: |
;356/614-623 |
References Cited
[Referenced By]
U.S. Patent Documents
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3692414 |
September 1972 |
Hosterman et al. |
4136949 |
January 1979 |
Hayamizu et al. |
4744664 |
May 1988 |
Offt et al. |
5841539 |
November 1998 |
Ikurumi et al. |
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Foreign Patent Documents
Primary Examiner: Pham; Hoa Q.
Attorney, Agent or Firm: Bromberg & Sunstein LLP
Parent Case Text
The present invention application claims priority from U.S.
Provisional Patent Application Ser. No. 60/544,468, filed Feb. 13,
2004, and incorporated herein by reference.
Claims
What is claimed is:
1. An apparatus for non-contact characterization of a surface of a
body, the apparatus comprising: a. a stage for controlled
translation of the body in at least one direction; b. a non-contact
sensor characterized by a line of sight for measuring, at a
plurality of points in time, a distance from a specified point on
the surface of a body to a fiducial reference position and for
generating a signal characterizing the distance; and c. a carousel
characterized by an axis of rotation, the carousel having a
plurality of reflecting surfaces for varying the line of sight of
the non-contact sensor with respect to a specified plane in a
periodic manner, the plurality of reflecting surfaces disposed at
unequal angles with respect to the axis of rotation.
2. An apparatus in accordance with claim 1, wherein the stage is
capable of translation along orthogonal axes.
3. An apparatus in accordance with claim 1, wherein the stage is
capable of rotation about a specified axis.
4. An apparatus in accordance with claim 1, wherein the non-contact
sensor is a conoscopic probe.
5. An apparatus in accordance with claim 1, further comprising a
processor for receiving the signal corresponding to the distance
measured by the non-contact sensor at the plurality of points in
time and for reconstructing the surface of the body in a frame of
reference fixed with respect to the body.
6. A method for non-contact characterization of a surface of a
body, the method comprising: a. supporting the body on a stage; b.
translating the body in at least one direction; c. characterizing
distances from the surface of the body to a fiducial reference
along a periodic sequence of distinct lines of sight, the lines of
sight varying periodically in a direction parallel to a specified
plane while changing concurrently in a direction perpendicular to
the specified plane; and d. merging the distances to form an image
of the surface of the body.
7. A method in accordance with claim 6, further comprising:
characterizing distances to reference objects of known shape in
conjunction with translation of the body.
8. A method in accordance with claim 6, further comprising:
transforming each of the distances to a common frame of
reference.
9. A method in accordance with claim 6, wherein the step of
characterizing includes measuring the distances.
10. A method in accordance with claim 6, wherein the step of
characterizing distances from the surface of the body to a fiducial
reference includes scanning the body with a beam and with scanning
performed in non-orthogonal directions.
11. A method in accordance with claim 6, wherein the step of
characterizing distances from the surface of the body to a fiducial
reference includes measuring distances along a line of sight that
varies periodically with time.
12. A method in accordance with claim 6, wherein the step of
characterizing distances from the surface of the body to a fiducial
reference includes scanning the body with a beam of light reflected
from mirrors disposed upon a rotating carousel.
Description
FIELD OF THE INVENTION
The present invention relates to non-contacting metrology, and, in
particular, to an apparatus and methods for optical scanning and
digitizing the coordinates of a solid body of complex shape.
BACKGROUND ART
Coordinate measuring machines (CMMs) are employed to determine the
coordinates, in some specified frame of reference, of points on the
surface of a workpiece. CMMs are typically employed for digitizing
or imaging, whether for replicating a prototype, for example, or
for various manufacturing applications. The salient parts of a CMM
include a stage, or complex of stages, for moving the object to be
characterized, a probe for measuring the distance to a point on the
surface relative to a fiducial position, a control or computing
system, and measurement software for converting the measurement
into a meaningful format for the intended application.
Various probes may be employed for measuring a distance to the
surface of the measured workpiece. Any probe, currently known or
otherwise, is within the scope of the present invention.
One form of probe, typified by the ConoProbe.TM. supplied by
Optimet, Optical Metrology Ltd. of Jerusalem, Israel, employs
conoscopy, a form of holography. Conoscopy is an interferometric
technique capable of determining the distance to one or more points
on an object surface without employing a reference beam. Instead,
light emanating from a source region is prepared in a defined state
of polarization and then passed through an anisotropic optical
element in which one polarization suffers phase retardation with
respect to the other. The two polarization components emerging from
the anisotropic optical element interfere with one another,
producing a interferogram in the detector plane. Conoscopy is the
subject of various patents, including U.S. Pat. Nos. 4,602,844,
4,976,504, 5,081,540, 5,081,541, and 5,953,137, all of which
patents are incorporated herein by reference.
One limitation imposed by existing CMMs is due to the fact that
even the most versatile optical sensors are unable to digitize on
vertical or very steep angles measured with respect to the optical
axis (or `line of sight`) of the probe. `Vertical`, in this case,
refers to the surface of the scanned body lying parallel to the
optical axis of the probe. An `undercut` refers to a negative angle
relative to the line of sight. Some applications, however, such as
dental surface profiling for purposes of reconstruction,
orthodontics, etc., as well as digitization of plastic parts,
molds, etc., require measurements on vertical walls or low angle
undercuts.
As used herein, a body characterized as `complex` is one having
vertical walls or low angle undercuts. Using prior art technology,
scanning a complex body requires orthogonal scanning the object
about multiple (typically 5) axes. As used herein, `orthogonal
scanning` refers to scanning of the line of sight of a probe
entirely within a single plane normal to an axis of rotation. This
method, while algorithmically simple, requires very large travel on
the scanning stages making the equipment very expensive.
Another prior art solution to the problem of small (or zero, or
negative) angles with respect to the probe line of sight is to
perform non orthogonal scanning by using a 2-axis angular arm. In
this case the whole sensor is rotated, and both the complex arm and
the requisite large travel ranges add to the cost of such systems.
Yet another prior art solution for scanning complex bodies requires
changing the sample position to allow direct line of sight for each
feature. In this case very complex reconstruction software is
required to merge the individual scans by `best fit` of complex
surfaces. The results of the `stitch` depend on the quality of the
data, size and shape of common features used for reference and the
robustness of the algorithms. Typically, operator intervention is
required, both during the scanning (otherwise 2 or more motorized
axis are required on the sample fixture) and during data
processing. In some cases like dental applications or plastic parts
with smooth surface features it is very difficult to find the right
fit and reference items to register successive `views` of the
object. One solution is to `glue` registration features (`balls`)
to the sample.
Clearly, an automated and robust solution to the problem of
digitizing complex bodies is desirable.
SUMMARY OF THE INVENTION
In accordance with a preferred embodiment of the present invention,
there is provided an apparatus for non-contact characterization of
a surface of a body. The apparatus has a stage for controlled
translation of the body in at least one direction, a non-contact
sensor characterized by a line of sight for measuring a distance
from a specified point on the surface of a body to a fiducial
reference position, and a carousel having a plurality of reflectors
for varying the line of sight of the non-contact sensor with
respect to a specified plane in a periodic manner.
In accordance with alternate embodiments of the invention, the
carousel may be adapted for rotation about one or more axes. The
stage may be capable of translation along orthogonal axes, or may
also be capable of rotation about a specified axis. The non-contact
sensor may, more particularly, be a conoscopic probe.
In yet further embodiments of the invention, the apparatus also has
a processor for receiving the signal corresponding to the distance
measured by the non-contact sensor at the plurality of points in
time and for reconstructing the surface of the body in a frame of
reference fixed with respect to the body.
Additionally, the invention provides a method for non-contact
characterization of a surface of a body. The method has steps of:
a. supporting the body on a stage; b. translating the body in at
least one direction; c. characterizing distances from the surface
of the body with respect to a fiducial reference along periodic
sequence of distinct lines of sight; and d. merging the distances
to form an image of the surface of the body.
The method may have an additional step of transforming each of the
distances to a common frame of reference. The step of
characterizing distances may include measuring the distances, and
scanning the body with a beam and with scanning performed in
non-orthogonal directions. Characterizing the distances may also
include measuring distances along a line of sight that varies
periodically with time, and scanning the body with a beam of light
reflected from mirrors disposed upon a rotating carousel.
The coordinates of reference objects of known shape may also be
determined in conjunction with translation of the body; and the
distances determined with respect to the reference objects may be
used to infer accurate mirror positions for image construction and
correction of distortion.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing features of the invention will be more readily
understood by reference to the following detailed description taken
with the accompanying drawings, in which:
FIG. 1 is a cutaway view of a multiple angle scanning system
depicting the scanning of a spherical object in accordance with
embodiment of the present invention;
FIG. 2 is a perspective view of a multiple angle scanning system
for dental applications, in accordance with a further embodiment of
the present invention;
FIG. 3A shows a circular profile (sphere/cylinder) as measured in
non orthogonal coordinates (laser beam bent normal to main scanning
direction) and
FIG. 3B shows the profile of FIG. 3A after transformation to
orthogonal coordinates;
FIG. 4 shows an image derived from 5 different surface scans taken
with 5 mirror positions after profile reconstruction in accordance
with embodiments of the present invention; and
FIG. 5 is a depiction of the scans shown in FIG. 4 after
coordinates translation using sphere fit (of the reference sphere)
for the translation.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
In preferred embodiments of the invention, a simple X-Y scanning
system is advantageously combined with a non contact sensor
characterized by an optical axis (or line of sight) and a variable
position mirror that varies the line of sight with respect to a
scanned body in a manner advantageously suited to automatic
digitization of complex bodies.
Referring, first, to FIG. 1, a perspective view is shown of the
salient components of a scanning system in accordance with
preferred embodiments of the present invention. An object 6 to be
measured, here represented by a sphere, is disposed on carriage 12
of a coordinate measuring machine, designated generally by numeral
10. Reference bodies may also be disposed on carriage 12 (also
referred to as a `support`) so as to travel in synchrony with
object 6 as it is translated by the carriage. Two reference bodies
are shown in FIG. 1: a reference sphere 4 and an angular reference
V prism 5. Carriage 12 may be translated, along orthogonal axes, as
by means of orthogonal X-Y translation stages 7, however all means
of translating object 6 during the course of scanning is within the
scope of the present invention. Other means of moving carriage 12
along a known trajectory include motion along a vertical (Z) axis
and rotation about one or more specified axes.
A distance probe 1 is typically characterized by an optical axis or
line of sight, designated, in FIG. 1, by the line denoted by
numeral 14. In preferred embodiments of the invention, a laser beam
is emitted collinearly with optical axis 14 of the distance probe.
Distance probe 1 is a non-contact sensor such as a conoscopic
sensor, or any other distance probe.
Line of sight 14, and the laser beam coaligned with it, is bent by
one or more folding mirrors 3 so that the line of sight impinges
upon the surface of scanned body 6. It is to be understood that the
optical path shown is described for purposes of convenience and
that more complex optical paths, entailing any other optical
elements, are within the scope of the present invention and
described herein and as claimed in any appended claims.
In accordance with preferred embodiments of the invention, a
plurality of folding mirrors 3 are mounted on a multi-position
actuator 2 in such a way to allow laser beam bending in different
directions. Multi-position actuator 2 may be a carousel supporting
multiple mirrors that is rotated about a central axis 16. More
particularly, mirrors 3 may be mounted on a polygon, such as the
pentagon shown in FIG. 1. Mirrors 3 may be reflecting surfaces
fashioned in the carousel in a continuous or discontinuous sequence
of normal directions. The mirrors may also be mounted in preset
positions, as shown in FIG. 1. The normal directions of successive
reflecting surfaces are disposed at varying angles with respect to
the plane normal to the axis of rotation of multi-position actuator
2. Such disposition of reflective surfaces on the actuator 2 in
FIG. 1 is illustrated by unequal angles A and B between the
respective normals to the reflective surfaces of two adjacent
mirrors 3 and the central axis 16. Thus, as the actuator is
rotated, the line of sight of the distance probe does not sweep out
a plane but varies over a range of angles, typically on the order
of .+-.20.degree. with respect to the orthogonal plane.
Measurements of distances to the surface of body 6 along line of
sight 14 are collected and processed separately for each of the
mirrors 3, thus forming a distinct `cloud` of points attributable
to that mirror. Merger (or `stitching`) of the respective clouds of
points to form a single consistent image is discussed below.
In the case where mirrors 3 are disposed at preset angles, after
assembly an accurate measurement of each bending angle is performed
for each position of the actuator. The measured angle values are
transferred to the software and used for the coordinate
transformation from a coordinate system based on motion of the
scanning system 7 to an orthogonal coordinate system. (Since the
line of sight is incident onto the surface of the object at an
angle that is specific to each mirror position, a separate
coordinate system attaches to each position of the mirror until
coordinates are transformed to an orthogonal system.
Desired bending angles may be calculated for a particular
application in order to cover the desired undercut angle and leave
enough working range. Mirrors 3 may then be set accordingly. High
bending angles will reduce the actual working range relative to
vertical position and require larger travel for the moving
stages.
It is to be noted that the main scanning movement (typically X-Y)
may be performed either by the measured object and reference
samples fixed on a mounting table or, as a matter of design choice,
by the sensor mounted together with the folding mirrors on a common
support. Both equivalent motions are within the scope of the
present invention.
Data from the measurements described herein are advantageously
gathered and processed automatically and without human
intervention. In order to increase accuracy of measurement, a fixed
reference object (sphere 4, V prism 5) is scanned before the part
is scanned, enabling an accurate evaluation of the beam position
relative to the X-Y scanning system 7.
All the gathered data for each mirror position is processed first
by applying the angular correction for the profiles to orthogonal
coordinates and then an origin shift, using the spherical fit to
translate the local coordinates to the same origin in a frame of
reference fixed with respect to the body. Since the measured object
6 remains fixed (relative to the mounting device, carriage 12) for
the entire measurement cycle, the actual X, Y coordinates of each
point are the same for all beam positions, thus simple robust
mathematical methods are advantageously employed for data
processing in a totally automatic cycle.
Referring now to FIG. 2, a coordinate measuring employing
embodiment of the present invention is shown, with components
labeled as described above with reference to FIG. 1. The scanned
object 6 in this case is a dental mold. Also shown are base 20 and
controller box 23 containing a processor for control of motion of
stage actuators 7 and analysis of clouds of points from each mirror
position and merger of the separate clouds into one single cloud
that reflects the actual shape of the 3D object.
The apparatus and methods described herein are advantageously
applied to collinear, non-contact measurement sensors that require
simple, trigonometric based transformation algorithms. A complete
or partial reconstruction of a 3D object may be derived, 360
degrees of a sphere with a 3-axis X, Y, Z/rotating axis and one
axis mirror drive or X-Y system and 2 axis mirror drive, or 270
degrees of a sphere with a 2 axis X-Y and one axis mirror
drive.
In accordance with alternate embodiments of the invention,
non-orthogonal scanning with non contact sensor may be applied to
common X-Y CMM scanning machines, with optional Z translation or
rotation about specified axes.
After setup in which sample borders are defined in software, data
gathering proceeds automatically. Trigonometric matrix
transformations of the non orthogonal coordinates and a sphere fit
provide for relative coordinate origin translation, transforming
the raw data of FIG. 3A to the spherical fit of FIG. 3B.
Point clouds due to distinct partial scans, as shown in FIG. 4, are
merged, after coordinate transformation and translation to the
merged point cloud shown in FIG. 5.
The described embodiments of the inventions are intended to be
merely exemplary and numerous variations and modifications will be
apparent to those skilled in the art. All such variations and
modifications are intended to be within the scope of the present
invention as defined in the appended claims.
* * * * *